This invention relates to a male-sterile plant and to its reproduction material (e.g., seeds), in which the cells are transformed so that a foreign DNA sequence is stably integrated into their nuclear genome. The foreign DNA sequence of this invention contains at least one first foreign DNA (hereinafter the xe2x80x9cmale-sterility DNAxe2x80x9d) that: 1) sequence a first RNA or protein or polypeptide which, when produced or overproduced in a stamen cell of the plant, disturbs significantly the metabolism, functioning and/or development of the stamen cell; and 2) is in the same transcriptional unit as, and under the control of, a first promoter which is capable of directing expression of the male-sterility DNA selectively in stamen cells of the plant. In particular, this invention relates to such a nuclear male-sterile plant and its reproduction material, in which the foreign DNA sequence of this invention is a foreign chimaeric DNA sequence that can also contain at least one second foreign DNA (the xe2x80x9cmarker DNAxe2x80x9d) that: 1) encodes a second RNA or protein or polypeptide which, when present at least in a specific tissue or specific cells of the plant, renders the entire plant easily separable from other plants that do not contain the second RNA, protein or polypeptide at least in the specific tissue or specific cells; 2) is in the same transcriptional unit as, and under the control of, a second promoter which is capable of directing expression of the marker DNA in at least the specific tissue or the specific cells of the plant; and 3) is in the same genetic locus of the nuclear genome of the cells of the plant as the male-sterility DNA.
This invention also relates to a foreign chimaeric DNA sequence that contains at least one male-sterility DNA under the control of the first promoter and that can also contain, adjacent to the male-sterility DNA, at least one marker DNA under the control of the second promoter.
This invention further relates to a vector that contains the foreign DNA sequence of this invention and is suitable for the transformation of plant cells, whereby the foreign DNA sequence is stably integrated into the nuclear genome of the cells.
This invention still further relates to cells of a plant and to plant cell cultures, the nuclear genomes of which are transformed with the foreign DNA sequence.
This invention yet further relates to a process for producing a nuclear male-sterile plant and its reproduction material and its cell cultures containing the foreign DNA sequence in which the male-sterility DNA: 1) is under the control of the first promoter and optionally in the same genetic locus as the marker DNA under the control of the second promoter; 2) is stably integrated into the nuclear genome of the plant""s cells; and 3) can be expressed selectively in stamen cells of the plant in the form of the first RNA, protein or polypeptide.
The invention further relates to a process for producing hybrid seeds, which grow into hybrid plants, by crossing: 1) the male-sterile plant of this invention which includes, in its nuclear genome, the marker DNA, preferably encoding a protein conferring a resistance to a herbicide on the plant; and 2) a male-fertile plant without the marker DNA in its genome. This invention particularly relates to such a process for producing hybrid seeds on a commercial scale, preferably in a substantially random population, without the need for extensive hand-labor.
This invention still further relates to a tapetum-specific promoter from a plant genome. This promoter can be used as the first promoter in the foreign DNA sequence of this invention for transforming the plant to render it nuclear male-sterile.
Hybridization of plants is recognized as an important process for producing offspring having a combination of the desirable traits of the parent plants. The resulting hybrid offspring often have the ability to outperform the parents in different traits, such as in yield, adaptability to environmental changes, and disease resistance. This ability is called xe2x80x9cheterosisxe2x80x9d or xe2x80x9chybrid vigorxe2x80x9d. As a result, hybridization has been used extensively for improving major crops, such as corn, sugarbeet and sunflower. For a number of reasons, primarily related to the fact that most plants are capable of undergoing both self-pollination and cross-pollination, the controlled cross-pollination of plants without significant self-pollination, to produce a harvest of hybrid seeds, has been difficult to achieve on a commercial scale.
In nature, the vast majority of crop plants produce male and female reproductive organs on the same plant, usually in close proximity to one another in the same flower. This favors self-pollination. Some plants, however, are exceptions as a result of the particular morphology of their reproductive organs which favors cross-pollination. These plants produce hybrid offspring with improved vigor and adaptability. One such morphology in Cannabis ssp. (hemp) involves male and female reproduction organs on separate plants. Another such morphology in Zea mays (corn) involves male and female reproductive organs on different parts of the same plant. Another such morphology in Elaeis guineensis (oilpalm) involves male and fertile female gametes which become fertile at different times in the plant""s development.
Some other plant species, such as Ananas comosus (pineapple), favor cross-pollination through the particular physiology of their reproductive organs. Such plants have developed a so-called xe2x80x9cself-incompatibility systemxe2x80x9d whereby the pollen of one plant is not able to fertilize the female gamete of the same plant or of another plant with the same genotype.
Some other plant species favor cross-pollination by naturally displaying the so-called genomic characteristic of xe2x80x9cmale sterilityxe2x80x9d. By this characteristic, the plants"" anthers degenerate before pollen, produced by the anthers, reach maturity. See: xe2x80x9cMale-Sterility in Higher Plantsxe2x80x9d, M. L. H. Kaul, 1987, in: Monographs on Theoretical and Applied Genetics 10, Edit. Springer Verlag. Such a natural male-sterility characteristic is believed to result from a wide range of natural mutations, most often involving recessive deficiencies, and this characteristic can not easily be maintained in plant species that predominantly self-pollinate, since under natural conditions, no seeds will be produced.
There are four main types of male sterility observed in nature. All four types of male sterility are used in commercial breeding programs to ensure that there is cross-pollination to produce hybrid seed for crops such as corn, sugarbeet, oilseed rape and sunflower.
One type of male sterility is nuclear encoded and is believed to be inherited as a recessive allele. For breeding purposes, a recessive male-sterile parent plant is maintained by crossing it with a heterozygous male-fertile plant that also includes the recessive male-sterility allele, so that the offspring are 50% recessive male-sterile plants. The other 50% are male-fertile plants that have to be rogued out in outcrossing programs which can only be done efficiently if the recessive male-sterility allele is segregated together with a selectable or screenable marker. In U.S. Pat. No. 4,727,219, a procedure is described for the use of recessive male sterility for the production of hybrid maize.
A second type of male sterility is nuclear encoded but inherited as a dominant allele. An advantage of dominant male sterile plants, as compared to recessive male sterile plants, is that the dominant male-sterile plants can be maintained through crossing with a male-fertile plant, to produce offspring that are 50% dominant male-sterile plants. The usefulness of this dominant nuclear male-sterile plant is, however, limited because its dominant male-sterility allele is in most cases not tightly linked (i.e., within the same genetic locus) to a selectable or screenable marker.
A third type of male sterility is cytoplasmatically encoded. In most cases, the cytoplasmic code is in the mitochondrial genome of the plant, and only in a few cases is the code in the chloroplast genome of the plant. The inheritance of cytoplasmatically encoded male sterility does not follow Mendelian rules but rather depends on cytoplasmic factors. The offspring obtained from crosses between cytoplasmic male-sterile plants and male-fertile plants all carry the cytoplasmic male-sterility gene and are therefore sterile. As a result, the offspring of plants of this type are only of commercial value if the economic product of the offspring is not for use as seed but rather for plants such as ornamentals and sugarbeet.
A fourth type of male sterility is the result of a combination of both nuclear encoded male sterility and cytoplasmatically encoded male sterility. The male sterility-inducing nuclear alleles are usually recessive, and only plants that contain the male-sterility cytoplasmic allele and that are homozygous for the male sterility-inducing nuclear allele are phenotypically male sterile. In this type of plant, corresponding dominant male fertility-inducing alleles or xe2x80x9crestorers of fertilityxe2x80x9d, produce a male-fertile phenotype. As a result, the male-sterile offspring of this type of plant can be made male-fertile by pollinating the male-sterile plants with pollen containing the restorers of fertility. As a result, the offspring of plants of this type are of commercial value where the economic product is seed, that is for plants such as corn, sorghum and sunflower.
Typically, hybrid seed production has been accomplished by the large scale planting of cytoplasmic male-sterile plants and male-fertile plants and by somehow (e.g., with a distinctive marker) preventing the resulting hybrid seeds from becoming mixed with non-hybrid seeds. According to U.S. Pat. No. 3,842,538, hybrid seeds are tediously separated from non-hybrid seeds on the basis of color. According to U.S. Pat. No. 4,351,130, the problem of separating hybrid seeds from non-hybrid seeds is avoided by using short male-sterile plants and tall male-fertile plants and then destroying the tall male-fertile plants after pollination. According to U.S. Pat. Nos. 4,658,085, 4,517,763 and 4,658,084, cytoplasmic male-sterile plants are provided with a herbicide tolerance absent from the male-fertile plants which are destroyed with the herbicide after pollination. According to U.S. Pat. No. 4,305,225, male-sterile rice plants are sprayed with a growth hormone (e.g., gibberellin) in order to cause fuller emergence of flower-bearing panicles from rice leaf sheaths, thereby increasing the ability of the flowers to receive pollen from male-fertile plants.
In all such processes for producing hybrid seeds from male-sterile plants, ways have been sought for simply and inexpensively obtaining on a commercial scale: 1) high hybrid seed production from each male-sterile plant; 2) a hybrid seed population that results almost exclusively from pollen of male-fertile plants and eggs of male-sterile plants and is substantially free of non-hybrid seeds from male-fertile plants; 3) easy production of both the male-sterile and male-fertile plants; and 4) the virtually complete removal or destruction of either the male-fertile plants after they have pollinated the male-sterile plants or the selective separation of non-hybrid seeds, produced by the male-fertile plants, from the hybrid seeds produced by the male-sterile plants.
In accordance with this invention, a cell of a plant is provided, in which the nuclear genome is transformed with a foreign DNA sequence, preferably a foreign chimaeric DNA sequence, characterized by:
(a) a male-sterility DNA encoding a first RNA, protein or polypeptide which, when produced or overproduced in a stamen cell of the plant, disturbs significantly the metabolism, functioning and/or development of the stamen cell; and
(b) a first promoter capable of directing expression of the male-sterility DNA selectively in stamen cells of the plant; the male-sterility DNA being in the same transcriptional unit as, and under the control of, the first promoter.
The foreign DNA sequence in the nuclear genome of the transformed cell can also comprise, preferably in the same genetic locus as the male-sterility DNA:
(c) a marker DNA encoding a second RNA, protein or polypeptide which, when present at least in a specific tissue or specific cells of the plant, renders the plant easily separable from other plants which do not contain the second RNA, protein or polypeptide at least in the specific tissue or specific cells; and
(d) a second promoter capable of directing expression of the marker DNA at least in the specific tissue or specific cells; the marker DNA being in the same transcriptional unit as, and under the control of, the second promoter.
Also in accordance with this invention is provided a foreign chimaeric DNA sequence that comprises the male-sterility DNA and the first promoter and that can also comprise the marker DNA and the second promoter, as well as at least one additional DNA encoding a transit peptide capable of transporting the first protein or polypeptide or the second protein or polypeptide into a chloroplast or mitochondria of a plant cell in which the foreign chimaeric DNA sequence is expressed in its cytoplasm.
Further in accordance with this invention are provided: a male-sterile plant and a plant cell culture, each consisting of the transformed cells; a seed of the male-sterile plant; hybrid seeds and plants produced by crossing the male-sterile plant with a male-fertile plant; and a process for producing such hybrid seeds.
Still further in accordance with this invention are provided tapetum-specific first promoters.
In accordance with this invention, a male-sterile plant is produced from a single cell of a plant by transforming the plant cell in a well known manner to stably insert, into the nuclear genome of the cell, the foreign DNA sequence of this invention. The foreign DNA sequence comprises at least one male-sterility DNA that is under the control of, and fused at its 5xe2x80x2 end to, the first promoter and is fused at its 3xe2x80x2 end to suitable transcription regulation signals (including a polyadenylation signal). Thereby, the first RNA, protein or polypeptide is produced or overproduced selectively in stamen cells of the plant so as to render the plant male-sterile. Preferably, the foreign DNA sequence also comprises at least one marker DNA that is under the control of, and is fused at its 5xe2x80x2 end to, the second promoter and is fused at its 3xe2x80x2 end to suitable transcription regulation signals (including a polyadenylation signal). The marker DNA is preferably in the same genetic locus as the male-sterility, whereby the second RNA, protein or polypeptide is produced in at least the specific tissue or specific cells of the plant so that the plant can be easily distinguished and/or separated from other plants that do not contain the second RNA, protein or polypeptide in the specific tissue or specific cells. This guarantees, with a high degree of certainty, the joint segregation of both the male-sterility DNA and the marker DNA into offspring of the plant.
The cell of a plant (particularly a plant capable of being infected with Agrobacterium) is preferably transformed in accordance with this invention, using a vector that is a disarmed Ti-plasmid containing the foreign DNA sequence and carried by Agrobacterium. This transformation can be carried out using procedures described, for example, in European patent publications 0,116,718 and 0,270,822. Preferred Ti-plasmid vectors contain the foreign DNA sequence between the border sequences, or at least located to the left of the right border sequence, of the T-DNA of the Ti-plasmid. Of course, other types of vectors can be used to transform the plant cell, using procedures such as direct gene transfer (as described, for example, in European patent publication 0,223,247), pollen mediated transformation (as described, for example, in European patent publication 0,270,356, PCT publication WO85/01856, and European patent publication 0,275,069), in vitro protoplast transformation (as described, for example, in U.S. Pat. No. 4,684,611), plant RNA virus-mediated transformation (as described, for example, in European patent publication 0,067,553, and U.S. Pat. No. 4,407,956) and liposome-mediated transformation (as described, for example, in U.S. Pat. No. 4,536,475).
Preferably, a nuclear male-sterile plant of this invention is provided by transforming a plant cell with a disarmed Ti-plasmid vector containing the foreign DNA sequence with both a male-sterility DNA under the control of a first promoter and a marker DNA under the control of a second promoter. The marker DNA can be upstream or downstream of the male-sterility DNA in the Ti-plasmid vector, but preferably, the two are adjacent to one another and are located between the border sequences or at least located to the left of the right border sequence of the Ti-plasmid vector, so that they are properly transferred together into the nuclear genome of the plant cell. However, if desired, the cell can initially be transformed with a foreign DNA sequence containing a male-sterility DNA and a first promoter and can subsequently be transformed with a marker DNA and a second promoter, inserted into the same genetic locus in the cell""s nuclear genome as the male-sterility DNA. Suitable vectors for this purpose are the same as those discussed above for transforming cells with the foreign DNA sequence. The preferred vector is a disarmed Ti-plasmid vector.
The selection of the male-sterility DNA is not critical. A suitable male-sterility DNA can be selected and isolated in a well-known manner, so that it encodes the first RNA, protein or polypeptide which significantly disturbs the proper metabolism, functioning and/or development of any stamen cell in which the male-sterility DNA is expressed, preferably leading thereby to the death of any such stamen cell. Preferred examples of male-sterility DNAs encode: RNases such as RNase T1 (which degrades RNA molecules by hydrolyzing the bond after any guanine residue) and Barnase; DNases such as an endonuclease (e.g., EcoRI); or proteases such as a papain (e.g., papain zymogen and papain active protein).
Other examples of male-sterility DNAs encode enzymes which catalyze the synthesis of phytohormones, such as: isopentenyl transferase which is an enzyme that catalyzes the first step in cytokinin biosynthesis and is encoded by gene 4 of Agrobacterium T-DNA; and the enzymes involved in is the synthesis of auxin and encoded by gene 1 and gene 2 of Agrobacterium T-DNA. Yet other examples of male-sterility DNAs encode: glucanases; lipases such as phospholipase A2 (Verheij et al (1981) Rev. Biochem. Pharmacol. 91, 92-203); lipid peroxidases; or plant cell wall inhibitors. Still other examples of male-sterility DNAs encode proteins toxic to plants cells, such as a bacterial toxin (e.g., the B-fragment of diphtheria toxin or botulin).
Still another example of a male-sterility DNA is an antisense DNA which encodes a strand of DNA complementary to a strand of DNA that is naturally transcribed in the plant""s stamen cells under the control of an endogenous promoter as described, for example, in European patent publication 0,223,399. Such an antisense DNA can be transcribed into an RNA sequence capable of binding to the coding and/or non-coding portion of an RNA, naturally produced in the stamen cell, so as to inhibit the translation of the naturally produced RNA. An example of such an antisense DNA is the antisense DNA of the TA29 gene (described in Example 2) which is naturally expressed, under the control of the TA29 promoter, in tapetum cells of the anthers of plants.
A further example of a male-sterility DNA encodes a specific RNA enzyme (i.e., a so-called xe2x80x9cribozymexe2x80x9d), capable of highly specific cleavage against a given target sequence, as described by Haseloff and Gerlach (1988) Nature 334, 585-591. Such a ribozyme is, for example, the ribozyme targeted against the RNA encoded by the TA29 gene.
Still other examples of male-sterility DNAs encode products which can render the stamen cells susceptible to specific diseases, such as fungus infections. Such a male-sterility DNA can be used in a plant wherein all other cells, in which the male-sterility DNA is not expressed, are resistant to the specific disease.
By xe2x80x9cforeignxe2x80x9d with regard to the foreign DNA sequence of this invention is meant that the foreign DNA sequence contains a foreign male-sterility DNA and/or a foreign first promoter. By xe2x80x9cforeignxe2x80x9d with regard to a DNA, such as a male-sterility DNA and a first promoter, as well a marker DNA, a second promoter and any other DNA in the foreign DNA sequence, is meant that such a DNA is not in the same genomic environment in a plant cell, transformed with such a DNA in accordance with this invention, as is such a DNA when it is naturally found in the cell of the plant, bacteria, animal, fungus, virus, or the like, from which such a DNA originates. This means, for example, that a foreign male-sterility DNA or marker DNA can be: 1) a nuclear DNA in a plant of origin; 2) endogenous to the transformed plant cell (i.e., from a plant of origin with the same genotype as the plant being transformed); and 3) within the same transcriptional unit as its own endogenous promotor and 3xe2x80x2 end transcription regulation signals (from the plant of origin) in the foreign DNA sequence of this invention in the transformed plant cell; but 4) inserted in a different place in the nuclear genome of the transformed plant cell than it was in the plant of origin so that it is not surrounded in the transformed plant cell by the genes which surrounded it naturally in the plant of origin. A foreign male-sterility or marker DNA can also, for example, be: 1) a nuclear DNA in a plant of origin; and 2) endogenous to the transformed plant cell; but 3) in the same transcriptional unit as a different (i.e., not its own) endogenous promotor and/or 3xe2x80x2 end transcription regulation signals in a foreign chimaeric DNA sequence of this invention in a transformed plant cell. A foreign male-sterility or marker DNA can also, for example, be: 1) a nuclear DNA in a plant of origin; and 2) endogenous to the transformed plant cell; but 3) in the same transcriptional unit as a heterologous promotor and/or 3xe2x80x2 end transcription regulation signals in a foreign chimaeric DNA sequence of this invention in a transformed plant cell. A foreign male-sterility or marker DNA can also, for example, be heterologous to the transformed plant cell and in the same transcriptional unit as an endogenous promotor and/or 3xe2x80x2 transcription regulation signals (e.g., from the nuclear genome of a plant with the same genotype as the plant being transformed) in a foreign chimaeric DNA sequence of this invention in a transformed plant cell. An example of a foreign male-sterility DNA could come from the nuclear genome of a plant with the same genotype as the plant being transformed and encode a catalytic enzyme, such as a protease or ribonuclease, that is endogenous to stamen cells of the plant being transformed, so that the enzyme is overproduced in transformed stamen cells in order to disturb significantly their metabolism, functioning and/or development. Preferably, the male-sterility DNA and the marker DNA are each heterologous to the plant cell being transformed.
By xe2x80x9cheterologousxe2x80x9d with regard to a DNA, such as a male-sterility DNA, a first promoter, a marker DNA, a second promoter and any other DNA in the foreign DNA sequence, is meant that such a DNA is not naturally found in the nuclear genome of cells of a plant with the same genotype as the plant being transformed. Examples of heterologous DNAs include chloroplast and mitochondrial DNAs obtained from a plant with the same genotype as the plant being transformed, but preferred examples are chloroplast, mitochondrial, and nuclear DNAs from plants having a different genotype than the plant being transformed, DNAs from animal and bacterial genomes, and chromosomal and plasmidial DNAs from fungal and viral genomes.
By xe2x80x9cchimaericxe2x80x9d with regard to the foreign DNA sequence of this invention is meant that at least one of its male-sterility DNAs: 1) is not naturally found under the control of its first promoter for the one male-sterility DNA; and/or 2) is not naturally found in the same genetic locus as at least one of its marker DNAs. Examples of foreign chimaeric DNA sequences of this invention comprise: a male-sterility DNA of bacterial origin under the control of a first promoter of plant origin; and a male-sterility DNA of plant origin under the control of a first promoter of plant origin and in the same genetic locus as a marker DNA of bacterial origin.
So that the male-sterility DNA is expressed selectively in stamen cells of a plant, it is preferred that the first promoter, which controls the male-sterility DNA in the foreign DNA sequence, be a promoter capable of directing gene expression selectively in stamen cells of the plant. (By xe2x80x9cstamenxe2x80x9d is meant the organ of the flower that produces the male gamete and that includes an anther and a filament). Such a stamen-specific promoter can be an endogenous promoter or an exogenous promoter and can be from the nuclear genome or from the mitochondrial or chloroplast genome of a plant cell. In any event, the first promoter is foreign to the nuclear genome of the plant cell, being transformed. Preferably, the first promoter causes the male-sterility DNA to be expressed only in anther, pollen or filament cells, especially in tapetum or anther epidermal cells. The first promoter can be selected and isolated in a well known manner from the species of plant, to be rendered male-sterile, so that the first promoter directs expression of the male-sterility DNA selectively in stamen cells so as to kill or disable the stamen and render the plant incapable of producing fertile male gametes. The first promoter is preferably also selected and isolated so that it is effective to prevent expression of the male-sterility DNA in other parts of the plant that are not involved in the production of fertile pollen, especially in female organs of the plant. For example, a suitable endogenous stamen-specific first promoter can be identified and isolated in a plant, to be made male-sterile, by:
1. searching for an mRNA which is only present in the plant during the development of its stamen, preferably its anthers, pollen or filament;
2. isolating this stamen-specific mRNA;
3. preparing a cDNA from this stamen-specific mRNA;
4. using this cDNA as a probe to identify the regions in the plant genome which contain DNA coding for the stamen-specific mRNA; and then
5. identifying the portion of the plant genome that is upstream (i.e., 5xe2x80x2) from the DNA coding for the stamen-specific mRNA and that contains the promoter of this DNA.
Examples of such first promoters are the TA29 promoter, the TA26 promoter and the TA13 promoter, hereinafter described in the Examples, which have been isolated from tobacco and are tapetum-specific promoters. Another tapetum-specific first promoter from another plant species can be isolated from its genome, using the TA29, TA26 or TA13 gene as a probe as in step 4, above. Under hybridizing conditions, such a probe will hybridize to DNA coding for a tapetum-specific mRNA in a mixture of DNA sequences from the genome of the other plant species (Maniatis et al (1982) Molecular Cloning. A Laboratory Manual. Ed. Cold Spring Harbor Laboratory). Thereafter, as in step 5 above, the other tapetum-specific first promoter can be identified.
If more than one male-sterility DNA is present in the foreign DNA sequence of this invention, all the male-sterility DNAs can be under the control of a single first promoter, but preferably, each male-sterility DNA is under the control of its own separate first promoter. Where a plurality of male-sterility DNAs are present in the foreign DNA sequence, the male-sterility DNA also can encode the same or different first RNA(s), polypeptide(s) and protein(s). For example, when the male-sterility DNA encodes an RNase such as RNase T1, it preferred that at least 3, particularly 4 to 6, copies of the male-sterility DNA and its first promoter be provided in the foreign DNA sequence. In any event, all the male-sterility DNA(s) and their first promoter(s) are preferably adjacent to one another in the foreign DNA sequence and in any vector used to transform plant cells with the foreign DNA sequence.
The selection of the marker DNA also is not critical. A suitable marker DNA can be selected and isolated in a well known manner, so that it encodes a second RNA, protein or polypeptide that allows plants, expressing the marker DNA, to be easily distinguished and separated from plants not expressing the second RNA, protein or polypeptide. Examples of marker DNAs encode proteins that can provide a distinguishable color to plant cells, such as the A1 gene encoding dihydroquercetin-4-reductase (Meyer et al (1987) Nature 330, 677-678) and the glucoronidase gene (Jefferson et al (1988) Proc. Natl. Acad. Sci. USA (xe2x80x9cPNASxe2x80x9d) 83, 8447), or that provide a specific morphological characteristic to the plant such as dwarf growth or a different shape of the leaves. Other examples of marker DNAs confer on plants: stress tolerance, such as is provided by the gene encoding superoxide dismutase as described in European patent application 88/402222.9; disease or pest resistance such as is provided by a gene encoding a Bacillus thuringiensis endotoxin conferring insect resistance as described in European patent application 86/300291.1 or a gene encoding a bacterial peptide that confers a bacterial resistance as described in European patent application 88/401673.4
Preferred marker DNAs encode second proteins or polypeptides inhibiting or neutralizing the action of herbicides such as: the sfr gene and the sfrv gene encoding enzymes conferring resistance to glutamine synthetase inhibitors such as Biolaphos and phosphinotricine as described in European patent application 87/400,544.0; genes encoding modified target enzymes for certain herbicides that have a lower affinity for the herbicides than naturally produced endogenous enzymes, such as a modified glutamine synthetase as target for phosphinotricine as described in European patent publication 0,240,792 and a modified 5-enolpyruvylshikimate-3 phosphate synthase as a target for glyphosate as described in European patent publication 0,218,571.
The second promoter, which controls the marker DNA, can also be selected and isolated in a well known manner so that the marker DNA is expressed either selectively in one or more specific tissues or specific cells or constitutively in the entire plant, as desired depending on the nature of the second RNA, protein or polypeptide encoded by the marker DNA. For example, if the marker DNA encodes an herbicide resistance, it may be useful to have the marker DNA expressed in all cells of the plant, using a strong constitutive second promoter such as a 35S promoter (Odell et al (1985) Nature 313, 810-812), a 35S""3 promoter (Hull and Howell (1987) Virology 86, 482-493), the promoter of the nopaline synthetase gene (xe2x80x9cPNOSxe2x80x9d) of the Ti-plasmid (Herrera-Estrella (1983) Nature 303, 209-213) or the promoter of the octopine synthase gene (xe2x80x9cPOCSxe2x80x9d [De Greve et al (1982) J. Mol. Appl. Genet. 1 (6), 499-511]). If the marker DNA encodes a protein conferring disease resistance, it may be useful to have the marker DNA selectively expressed in wound tissue by using, for example, a TR promoter such as the TR1xe2x80x2 or TR2xe2x80x2 promoter of the Ti-plasmid (Velten et al (1984) EMBO J. 3, 2723-2730). If the marker DNA encodes a herbicide resistance, it may be useful to have the marker DNA selectively expressed in green tissue by using, for example, the promoter of the gene encoding the small subunit of Rubisco (European patent application 87/400,544.0). If the marker DNA encodes a pigment, it may be useful to have the marker DNA expressed in specific cells, such as petal cells, leaf cells or seed cells, preferably in the outside layer of the seed coat.
One can identify and isolate in a well known manner a tissue-specific second promoter for a plant to be rendered male-sterile and easily distinguishable from non-transformed plants by:
1. searching for an mRNA which is only present in the plant during the development of a certain tissue, such as its petals, leaves or seeds;
2. isolating this tissue-specific mRNA;
3. preparing a cDNA from this tissue-specific mRNA;
4. using this cDNA as a probe to identify the regions in the plant genome which contain DNA coding for the tissue-specific mRNA; and then
5. identifying the portion of the plant genome that is upstream from the DNA coding for the tissue-specific mRNA and that contains the promoter for said DNA.
If more than one marker DNA is present in the foreign DNA sequence of this invention, all the marker DNAs can be under the control of a single second promoter, but preferably, each marker DNA is under the control of its own separate second promoter. More preferably, each marker DNA is under the control of its own second promoter and encodes a different second RNA, protein or polypeptide, providing different distinguishable characteristics to a transformed plant. In any event, the marker DNA(s) and second promoter(s) should be adjacent to each other and to the one or more male-sterility DNAs contained in the foreign DNA sequence of this invention and in any vector used to transform plant cells with the foreign DNA sequence.
It is generally preferred that the first RNA, protein or polypeptide, encoded by the male-sterility DNA, interfere significantly with the stamen cells"" metabolism, functioning and/or development by acting in the cytoplasm or the nucleus of the stamen cells. However, when it is desired to have the first protein or polypeptide and/or of the second protein or polypeptide transported from the cytoplasm into chloroplasts or mitochondria of the cells of transformed plants, the foreign DNA sequence can further include an additional foreign DNA encoding a transit peptide. The additional DNA is between the male-sterility DNA and the first promoter if the first protein or polypeptide is to be so-transported and is between the marker DNA and the second promoter if the second protein or polypeptide is to be so-transported. By xe2x80x9ctransit peptidexe2x80x9d is meant a polypeptide fragment which is normally associated with a chloroplast or mitochondrial protein or subunit of the protein and is produced in a cell as a precursor protein encoded by the nuclear DNA of the cell. The transit peptide is responsible for the translocation process of the nuclear-encoded chloroplast or mitochondrial protein or subunit into the chloroplast or the mitochondria, and during such a process, the transit peptide is separated or proteolytically removed from the chloroplast or mitochondrial protein or subunit. One or more of such additional DNA""s can be provided in the foreign DNA sequence of this invention for transporting one or more first or second proteins or polypeptides as generally described in European patent applications 85/402,596.2 and 88/402,222.9 and in: Van den Broeck et al (1985) Nature 313, 358-363; Schatz (1987) Eur. J. of Bioch. 165, 1-6; and Boutry et al (1987) Nature 328, 340-342. An example of a suitable transit peptide for transport into chloroplasts is the transit peptide of the small subunit of the enzyme RUBP carboxylase (European patent application 85/402,596.2) and an example of a transit peptide for transport into mitochondria is the transit peptide of the enzyme Mn-superoxide dismutase (see Example 16).
In the foreign DNA sequence of this invention, 3xe2x80x2 transcription regulation signals can be selected among those which are capable of enabling correct transcription termination and polyadenylation of mRNA in plant cells. The transcription regulation signals can be the natural ones of the gene to be transcribed but can also be foreign or heterologous. Examples of heterologous transcription regulation signals are those of the octopine synthase gene (Gielen et al (1984) EMBO J. 3, 835-845) and the T-DNA gene 7 (Velten and Schell (1985) Nucleic Acids Research (xe2x80x9cNARxe2x80x9d) 13, 6981-6998).
Also in accordance with this invention, plant cell cultures, such as anther cell cultures, containing the foreign DNA sequence of this invention in which the first promoter effects expression of the male-sterility DNA at a given stage of pollen development, more especially after meiosis, can be used to regenerate homozygous dominant male-sterile plants (xe2x80x9cEfficient isolation of microspores and the production of microspore-derived embryos from Brassica napusxe2x80x9d, E. B. Swanson, M. P. Coumans, S. C. Wu, T. L. Barby and W. D. Beversdorf, Plant Cell Reports (1987) 6: 94-97).
Further in accordance with this invention, processes are provided for producing hybrid seeds which can be grown into hybrid plants. One process involves crossing a nuclear male-sterile plant including at least one marker DNA with a male-fertile plant without the marker DNA. Both male-sterile and male-fertile plants are planted in separate rows near to each other. Another process involves crossing a nuclear male-sterile plant including at least two different marker DNAs with a male-fertile plant including, in common, only one of the two different marker DNAs in a homozygous form. Both male-sterile and male-fertile parent plants can be grown in a substantially random population, increasing the chances of cross-pollination, without the need for precise planting patterns. The male-fertile parent plant can thereafter be easily removed from the population, using the distinctive trait encoded by the non-common marker DNA which is not possessed by the male-fertile parent plant. Preferably in this process, the non-common marker DNA in the male-sterile plant is under the control of a constitutive promoter and encodes a protein or polypeptide that renders the male-sterile plant resistant to a particular herbicide. The male-fertile plant can then be destroyed after cross-pollination, using the particular herbicide.
Plants, transformed with the male-sterility DNA, preferably with both the male-sterility DNA and the marker DNA encoding herbicide-resistance, stably integrated and transmissible throughout generations as dominant alleles in accordance with this invention, are alternatives to, and provide several advantages over, presently used cytoplasmic male-sterility systems for breeding and producing hybrid crops. Such advantages include:
1. For cross-pollinating crops, the breeding strategy is much simplified, because it is not necessary to introduce a restorer gene into the male-fertile parent line of the cross that will produce the commercially sold hybrid seed. Indeed, a heterozygous nuclear male-sterile parent line crossed with another male-fertile parent line for commercial seed production will produce 50% male-sterile hybrid offspring and 50% male-fertile hybrid offspring, as a result of which the commercial crop will produce enough pollen to guarantee full seed set and therefore normal yield. Examples for such crops are corn and oilseed rape.
2. For crops for which the seeds do not represent the economic harvest, the breeding strategy is also much simplified without the need of a restorer gene expressed in the male-fertile parent line. Indeed, for these crops it does not matter that 50% of the commercially sold hybrid seeds are male-sterile. Examples for these crops are sugarbeet and alfalfa.
3. The system allows production of nuclear male-sterile lines and maintainer lines from existing inbred lines in one operation, eliminating the need for backcrossing. This reduces the time lag between conception and commercialization of a hybrid by at least 6 to 8 generations. An example of a typical strategy for producing hybrid plants using as parent plant the plants having inserted and expressing the male-sterility DNA may consist of the following steps:
1) making test hybrids by hand, by crossing inbred lines, and testing for combining ability and selected characteristics (2 years).
2) making one parent line of each of the selected hybrids nuclear male-sterile by the process which is the object of this invention (1 year).
3) multiplying the nuclear male sterile parent plant obtained from said process, hereinafter called xe2x80x9cASxe2x80x9d, and its maintainer line, hereinafter called xe2x80x9cAxe2x80x9d, and the pollinating male-fertile parent plant, hereinafter called xe2x80x9cBxe2x80x9d, of the future commercial crop (3 years). During the same period, introducing the selected hybrids in official yield trials (3 years).
4) producing and selling the approved hybrid seed (1 year).
4. Combined with a marker DNA encoding herbicide-resistance, such a nuclear male-sterility system allows production of 2-, 3- and 4-way hybrids in any combination required. It is believed to be sufficient to introduce the male-sterility DNA and adjacent thereto the marker DNA into the nuclear genome of one plant which will be used as one of the grandparent breeding lines for obtaining 2- or 3-way hybrids, and into the nuclear genome of two plants which will be used as the two grandparent lines for 4-way hybrids. Each breeding line can be maintained by the following two crosses given here by way of example, and whereby xe2x80x9cSHxe2x80x9d stands for the dominant alleles respectively of male-sterility (S) and herbicide resistance (H), and sh stands for the recessive alleles respectively of male fertility (s) and herbicide sensitivity (h):
a. SH/sh x sh/sh gives 50% SH and 50% sh offspring, and after spraying with the herbicide to which H confers resistance, 100% sterile seedlings are obtained.
b. sh/sh x sh/sh gives 100% fertile offspring.
5. It provides a protection for the owner of the marker DNA that has been integrated into the male-sterility system by making it more difficult for competitors to breed the marker DNA into their own breeding lines.
For illustrative purposes, two crop breeding schemes in accordance with this invention are given as follows:
1A) Maintaining the Male-sterility Line AS:
line ASH/sh x line Ash/sh giving 50% ASH/sh (phenotype: male-sterile, herbicide-resistant) 50% Ash/sh (phenotype: male-fertile, herbicide-susceptible)
1B) Producing the Hybrid Seed Crop:
a) planting seeds of Bsh/sh (male plants) and the seeds obtained by the cross 1A) consisting of ASH/sh and Ash/sh (xe2x80x9cfemalexe2x80x9d plants) in separate rows.
b) eliminating the genotype Ash/sh by spraying the female rows with the herbicide.
c) cross-pollination occurring: ASH/sh x Bsh/sh and Bsh/sh x Bsh/sh giving in the female rows: 50% ABSH/sh (phenotype: hybrid, male-sterile, herbicide-resistant) 50% ABsh/sh (phenotype: hybrid, male-fertile, herbicide-sensitive) and in the male rows: 100% Bsh/sh.
d) eliminating the genotype Bsh/sh occurring in the male rows by spraying with the herbicide or by mechanical means.
e) harvesting the hybrid seeds of the female rows wherein the cross-pollination of c) occurred This is the commercially sold seed.
2A) Maintaining the Male-sterile Line AS:
AS:ASH1H2/sh1h2 x Ash1h2/sh1h2 giving 50% ASH1H2/sh1h2 (phenotype: male-sterile, resistant to both herbicides). 50% Ash1h2/sh1h2 (phenotype: male-fertile, susceptible to both herbicides).
2B) Maintaining Pollination Line B:
Bsh1H2/sh1H2 x Bsh1H2/sh1H2 giving 100% Bsh1H2/sh1H2 (phenotype: male-fertile, susceptible to herbicide 1 and resistant to herbicide 2).
2C) Producing the Hybrid Seed Crop:
a) planting the seeds obtained from 2A) and the seeds obtained from 2B) at random.
b) eliminating the genotype Ash1h2/sh1h2 by spraying the field with herbicide 2.
c) cross-pollination occurring:
ASH1H2/sh1h2 x Bsh1H2/sh1H2 giving 50% ABSH1H2/sh1H2 50% ABsh1h2/sh1H2 and self-pollination occurring:
Bsh1H2/sh1H2 x Bsh1H2/sh1H2 giving 100% Bsh1H2/sh1H2 
d) eliminating plants with genotype Bsh1H2/sh1H2 obtained from the parent line B, for which self-pollination occurred, by spraying the field with herbicide 1.
e) harvesting hybrid seeds of the remaining plants ASH1H2/sh1H2 obtained by the cross-pollination of c).